Beverage bottles such as PET bottles are integrally provided with a tamper-proof ring, which is disposed usually at the lower end of cap skirt and is connected thereto through the breaking pieces that can be broken with the cap movement, so that the bottle can be prevented from being opened falsely in the shop.
This ring has some locking pieces on the inner wall. When the cap is turned in the uncapping direction and is screwed upward from the bottle, a stop ring disposed on the outer wall of the bottleneck blocks the locking pieces. At this point, the ring is prevented from rising, but since the climb of the cap is continued, the breaking pieces between the cap and the ring are broken to separate the cap from the ring. By checking on this separation, one can confirm whether the bottle has already been opened or not.
The cap is provided with a gasket on the inner wall of the cap to enhance air-tightness and to prevent the contents from running out and outside air from flowing into the bottle when the beverage bottle is under the sealed condition. This gasket is made of a properly elastic material and is pushed to the upper surface of the neck due to strong screw engagement between the cap and the bottleneck. Through this tight contact with the top surface of the neck, the gasket seals the opening of the neck.
However, in the case of those caps equipped with a tamper-proof ring, the force of pushing the gasket to the top surface of the neck may become weak with the upward displacement of the cap relative to the bottleneck. This occurs from a point at the limit of screw engagement till the time when the stop ring of the neck blocks the locking pieces of the ring and the breaking pieces start being broken. If a bottle cap is twisted in the shop within a limit in which the breaking pieces are not broken, the bottle looks normal on the exterior. Nevertheless, the sealing function of the gasket is actually in a decreased condition. In this state, the contents tend to be deteriorated, and great anxiety is stirred up in users.
A conceivable means of avoiding this inconvenience is to use the outer wall of the neck of the body as the seal surface. However, when the preform is injection-molded before it is biaxially drawn and blow molded into a bottle, a low, vertical ridge of flash 73 may be formed along the parting line over the entire height of the preform (See FIG. 8). If the outer wall of the neck were covered tightly with a gasket 74, an interstice 75 would be formed on both sides of the flash 73. As a result, the cap would have only a small cap-turning angle to maintain a no-leak condition, and there is a fear that the gasket cannot fully demonstrate its sealing function.
Another means of avoiding the inconvenience is shown in P1999-180458, which discloses that the inner wall of the bottleneck is used as the seal surface. That conventional art discloses a cap equipped with a tamper-proof ring, in which a cylindrical gasket support 54 is suspended from under the gasket 53 and is fitted deeply into the inner wall of the neck of the bottle 51. If the distance of tight contact is made longer between the gasket support 54 and the inner neck wall of the bottle 51, full air-tightness can be maintained even when the cap 52 is turned within a limit in which the breaking pieces 55 are not broken. (See FIG. 7)
However, in the above-described conventional art, the cylindrical gasket support 54 has an outer diameter larger than the bore diameter of the neck so that the gasket support 54 can be tightly fitted into the neck of the bottle 51. In addition, the gasket support 54 is molded so as to have a considerable height. At the time when the cap 52 is screwed on the neck, there may be misalignment on some occasions in the positions of the gasket 53 and the neck of the bottle 51. In that case, the gasket support 54 butts against the top surface of the neck, and then the gasket support 54 in a crooked state may be fitted into the neck, thus creating a problem that no air-tightness is available for the bottle.
This invention has been made to solve the above-described problem. The technical problem is to achieve air-tightness around the entire circumference between the gasket and the upper portion of the outer wall of the bottleneck. An object of this invention is to provide a synthetic resin bottle, in which air-tightness can be fully secured, and stable no-leak condition is maintained, up to a certain cap-turning angle even if the cap is falsely twisted. Another object of this invention is to obtain easily at an inexpensive cost the preform that is biaxially drawn and blow molded into this synthetic resin bottle.